In recent years, video printers for obtaining hard copies of images from video signals have been intensively developed. The images have been formed according to thermal transfer systems. More particularly, an ink ribbon comprised on a polyethylene terephthalate substrate and an ink layer formed by mixing a dye with or dissolving the dye in a hydrophobic polymer is provided. The ink layer is superposed in a dye-receiving hydrophobic polymer layer of a transfer material formed on a synthetic paper, under which the ink ribbon is heated according to image signals by means of a thermal head or the like. As a result, a disperse dye in the ink layer is thermally transferred to the dye-receiving layer to form an image. As the dyes of the thermal transfer ink ribbons which are employed in the thermal transfer system, sublimable dyes have been heretofore considered principally favorable from the standpoint of the image formation. According to recent investigative trends wherein types of materials have been taken into account, importance is placed on thermal diffusing properties rather than sublimability. Moreover, there are other important properties or factors of the dyes including miscibility with hydrophobic polymers used in the ink layer of the thermal transfer ink ribbon, dyeability against the dye-receiving layer consisting of hydrophobic polymers of the transfer material, and a degree of achievement of actual sensitivity at the time of the thermal transfer. To this end, disperse dyes have been frequently used as a dye for the thermal transfer ink ribbons.
However, when disperse dyes are used as the dye for thermal transfer ink ribbons, there arises the problem that they are unsatisfactory in practical utility from the standpoint of the sensitivity during the transfer operations and the hue and light fastness of the resultant images.
To avoid this, it may occur that cationic dyes which are known for dyeing acrylic fibers as having an inherent brightness, high coloring properties and good light fastness are used for the thermal transfer ribbons. Although the cationic dyes exhibit good light fastness and wet fastness, they are hydrophilic in nature, so that it is difficult to uniformly, stably keep the dye in butyryl resins ordinarily used as the binder of the ink layer of thermal transfer ink ribbons. Thus, it has not been possible for cationic dye to be used in the thermal transfer ink ribbon.